Water softening processes are often carried out for the removal of dissolved, suspended and colloidal solids from water for the purpose of rendering it more useful for household and/or industrial use. The softening of water is carried out by the removal of hardness ions either by resin or through precipitation. Precipitation is a result of exceeding the solubility of the molecule and is usually caused by heat or a change in pH. In many feedstream waters, acid gases exist that must also be removed. Silica, another common ion, is also desirably removed from the feedstream waters, often through the use of resins.
Lime softening water treatment processes may date back to antiquity. In the modern era, prior to the 1930's, water softening was carried out as a four step process. First, the hard water was mixed with lime, or lime plus soda ash, in a mixing tank. Secondly, precipitation and flocculation takes place in one or more flocculation chambers. Third, flocculated water then flows into a settling chamber where precipitated sludge is allowed to settle to the bottom and is drawn from the bottom of the settling tank for disposal. A portion of the sludge so removed is recycled to the mixing tank in order to speed the softening reactions. Finally, clarified water overflows from the top of the settling chamber and is filtered.
One significant advance in lime softening water treatment processes was the development as what is known as the spaulding contact reactor. This reactor incorporated the above-noted mixing, flocculation, precipitation, and settling steps in a single reactor structure. However, separate conventional through-flow filtration equipment is generally still required in conjunction with the spalding reactor. Spaulding reactors are generally large, complex, cumbersome and, hence, quite costly in terms of capital investment.
The most common methods of acid gas removal from water streams are gas or steam stripping with the addition of caustic, resins, or crystallization as the common methods of softening. Crystallization is usually accomplished through the addition of crystals and heat on a thin film column which promotes the formation of the crystals for removal.
In the past, techniques have not been developed whereby acid gas stripping, partial hardness removal, and silica reduction have occurred in a single process. Where such steps are required, these contaminants of the feedstream of water are removed through separate and isolated processes. The addition of resins and other cleansing compounds to the water is a very costly procedure. Additionally, after these resins are added to the water, there is no reuse of the resins.
U.S. Pat. No. 4,670,150, issued on June 2, 1987, to Hsing et al. describes a method for softening a fluid containing hardness constituents which includes the steps of: (1) mixing the fluid with sufficient amounts of a softening agent, and previously precipitated hardness constituents, in a reactor to establish a softening reaction in the reactor for precipitating a substantial portion of the hardness constituents contained in the fluid; (2) removing a portion of the fluid and precipitated hardness constituents from the reactor as a slurry and filtering the slurry through a cross-flow microfiltration module containing at least one elongated filtration tube, whereby substantially all of the precipitated hardness constituents are removed from the portion of the fluid exiting from the module as filtrate; and (3) returning at least a portion of the filtered out precipitated hardness constituents exiting from the module to the reactor as the previously precipitated hardness constituents.
U.S. Pat. No. 4,518,505, issued on May 21, 1985, to Lim et al. describes a process for heating and softening of hard water by direct heat exchange with steam. The steam is directly sparged into the hard water within a reaction zone and condensed in the water under a pressure sufficient to prevent boiling at the reaction zone temperature. The heated water is then drawn from the reaction zone without substantial vaporization of water and filter to remove scale precipitates resulting from the heating step. At least a portion of the softened water is heated to generate wet steam which is then recirculated to the reaction zone and sparged into the hard water therein.
The Lim patent describes a process for the condensing of steam rather than softening water by way of steam stripping. In the Lim patent, a high temperature must be used (374.degree. F. to 410.degree. F.) as the steam is condensed in order to convert the bicarbonate to carbonate. In the method of the Lim patent, it is very important to heat the water first, since the reaction is driven by the high temperature and not by the removal of carbon dioxide. No mechanism is provided in the Lim patent to strip the carbon dioxide. U.S. Pat. No. 2,401,924, issued on Jun. 11, 1946, to P.C. Goetz describes a process for the removal of silica from water. The Goetz patent describes the treatment of water for removing silica therefrom by utilizing a magnesium-rich sludge which is brought into an intimate, thorough and prolonged contact with the water while the water is at a temperature above about 50.degree. C. The sludge is obtained by the precipitation of calcium and magnesium salts from hard water by the addition thereto of lime and soda-if needed, or other suitable chemicals as in the hot lime soda process of water softening. In this patent, the magnesium hydroxide is used to adsorbing the silica. The majority of the silica is removed by magnesium silicate or a complex magnesium silicate scale. Some magnesium hydroxide can occur depending no the relative solubility constants. Silicate scales are less soluble than carbonate, hydroxides or sulfates.
It is an object of the present invention to provide a process for the softening of water that is effective for the reduction in hardness ions, silica, and acid gases.
It is another object of the present invention to provide a process which is more economically attractive than previous processes.
It is a further object of the present invention to provide a process for the softening of a feedstream of water that produces large energy savings.
It is another object of the present invention to provide a process for the conversion of bicarbonate to carbonate at temperatures below boiling.
It is another object of the present invention to provide a process which strips carbon dioxide from the feedstream of water.
It is a further object of the present invention to minimize heat and energy requirements of the softening process.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.